The present invention relates to an exhausts gas purification apparatus, an exhaust gas purification process and an exhaust gas purification catalyst with high elimination efficiency of NOx from NOx-containing exhaust gases such as combustion exhaust gases from internal combustion engines such as automobile engines, etc.
Lean-burn engines capable of bringing an air/fuel ratio into a fuel-lean stage have been nowadays regarded as promising automobile internal combustion engines from the viewpoint of reduced fuel consumption. However, the exhaust gases from such engines are in such an oxidizing atmosphere that the O2 concentration of the exhaust gases is more than a stoichiometric amount necessary for complete combustion of reducing components contained in the exhaust gases (said atmosphere will be hereinafter referred as xe2x80x9coxidizing atmospherexe2x80x9d). The conventional ternary catalysts are directed to efficient purification of NOx, HC and CO in such a reducing atmosphere that the O2 concentration of the exhaust gases is not more than a stoichiometric amount necessary for complete combustion of reducing components contained in the exhaust gases. (said atmosphere will be hereinafter referred to as xe2x80x9creducing atmospherexe2x80x9d), failing to show a satisfactory NOx elimination activity in the oxidizing atmosphere. Thus, it has been keenly desired to develop catalysts capable of effectively eliminating NOx, HC and CO, particularly NOx, in the oxidizing atmosphere.
For exhaust gas purification directed to the lean-burn engines, W093/07363 and W093/08383 propose to provide an NOx absorbent in the exhaust gas passage. The NOx absorbent can absorb NOx from the exhaust gases during the fuel-lean combustion and discharge the absorbed Nox when the O2 concentration of the exhaust gases is lowered.
JP-A-8-299793 also proposes to provide a catalyst which comprises an NOx-absorbing component capable of absorbing NOx from the exhaust gases during the fuel-lean combustion and an NOx-reducing component capable of reducing the absorbed NOx in the exhaust gas passage.
With more and more intensified atmospheric regulation of the automobile exhaust gases, much higher NOx elimination activity and durability are required for the Nox elimination catalysts directed to the lean-burn engines. The present invention provides an exhaust gas purification catalyst with distinguished NOx elimination activity and durability, particularly distinguished head resistance and SOx resistance, an exhaust gas purification apparatus and an exhaust gas purification process using said catalyst.
The present invention is directed to elimination of NOx in a combustion exhaust gas in an oxidizing atmosphere emitted from an internal combustion engine with a catalyst comprising a carrier and active components, the active components comprising at least one of Rh, Pt and Pd, at least one member selected from alkali metals and alkaline earth metals, and Mn.
One member selected from the alkali metals and alkaline earth metals can serve the desired purpose, but two or more members thereof can further improve the activity. It seems that new active sites are formed on the catalyst owing to two or more members of these metals supported on the carrier. An amount of supported alkali metals and alkaline earth metals is preferably in a range of 0.05 to 3 parts by mole each of the supported metals in terms of metal elements on the basis of 1.5 parts by mole of the porous carrier, where xe2x80x9cparts by molexe2x80x9d means a proportion of one component to another in terms of moles; for example, xe2x80x9c3 parts by mole of supported component B to 1.5 parts by mole of component Axe2x80x9d means that component B is supported in a ratio of component B to component A being 3:1.5 terms of moles, irrespective of the absolute amount of component A. In case the amount of supported alkali metals and alkaline earth metals is less than 0.05 parts by mole each, the improvement of NOx elimination activity is less effective, whereas in case of more than 3 parts by mole the specific surface area of the alkali metals and alkaline earth metals becomes undesirably smaller.
The porous carrier may be supported on a substrate, where 0.3 to 4 moles of the porous carrier can supported on 1 l of the substrate preferably from the viewpoint of NOx elimination activity. In case the amount of supported porous carrier is less than 0.3 moles, the dispersibility of active components becomes poor, whereas in case of more than 4 moles the specific surface area of the porous carrier itself becomes undesirably smaller.
Mn is present in the form of metal or an oxide or a composite oxide with Al, etc. and seems to serve to capture NOx in the oxidizing atmosphere and further serve to improve the high temperature durability of the catalyst. By inclusion of both of at least one member of the alkali metals and alkaline earth metals and Mn the NOx-capturing effect can be further improved.
An amount of supported Mn is preferably in a range of 0.05 to 2 parts by mole in terms of metal element on the basis of 1.5 parts by mole of the porous carrier. In case the amount of supported Mn is less than 0.05 parts by mole, the effect is not remarkable, whereas in case of more tan 2 parts by mole the specific surface area of the catalyst becomes undesirably smaller.
Rh, Pt and Pd can enhance the elimination activity and high temperature durability. It is most desirable for the improvement of the activity and durability to contain all of these noble metals.
Amounts of supported noble metals are preferably in ranges of 0.002 to 0.05 parts by mole of Pt, 0.0003 to 0.01 part by mole of Rh and 0.001 to 0.2 parts by mole of Pd, all in terms of metal elements, on the basis of 1.5 parts by mole of the porous carrier. In case the amount of supported noble metals are less than their lower limits the effect is not remarkable, whereas in case of more than the upper limits the specific surface areas of the noble metals per se become smaller without remarkable effect.
When at least one of rare earth metals is supported thereon in addition to the foregoing components, the NOx elimination activity and high temperature durability can be further improved, where it is preferable to contain 0.02 to 0.5 parts by mole of each in terms of metal elements on the basis of 1.5 parts by mole of the porous carrier. In case of less than 0.02 parts by mole, the effect is not remarkable, whereas in case of more than 0.5 parts by mole the specific surface area of the catalyst becomes undesirably smaller. Preferable rare earth metals are La, Nd and Ce.
By further adding at least one of Ti and Si thereto, the NOx elimination efficiency and also the SOx resistance can be further improved. It seems that the effect of Ti and Si on the SOx resistance improvement is due to Ti and Si being formed into composite metals together with Mn, alkali metals and alkaline earth metals. By further adding at least one of Co, Ni and Cu thereto, the NOx elimination activity and the heat resistance can be further improved. Amounts of supported Ti, Co, Si, Ni and Cu are preferably in a range of 0.01 to 2 parts by mole of each in terms of metal elements on the basis of 1.5 parts by mole of the porous carrier.
The present catalyst can further contain at least one of B and P.
B and P are present in the form of simple substances or oxides or composite oxides with at least one member selected from alkali metals and Al, and seem to serve to capture NOx in the oxidizing atmosphere, play a role of attracting CO, hydrocarbons, etc. as reducing agents onto the catalyst surface and serve to further improve the heat resistance and the SOx resistance of the catalyst. By controlling mixing sequence of B or P or firing temperature, etc. during the catalyst preparation, they can be brought into the oxide form or the composite oxide form.
An amount of supported B or P is preferably in a range of 0.01 to 2 parts by mole of each in terms of elements on the basis of 1.5 parts by mole of the porous carrier. In case of less than 0.01 part by mole of supported B or P the effect is not remarkable, whereas in case of more than 2 parts by mole the specific surface area of the catalyst becomes undesirably smaller.
The catalyst can be prepared by any procedure utilizing physical means or chemical reactions such as impregnation, kneading, coprecipitation, sol-gel formation, ion exchange, vapor deposition, etc.
Starting materials for use in the catalyst preparation include various compounds such as nitrate compounds, acetate compounds, complex compounds, hydroxides, carbonate compounds, organic compounds etc., metals and metal oxides.
For the porous carrier, metal oxides, composite oxides, etc. such as alumina, titania, silica, silica-alumina, zirconia, magnesia, etc. can be used, among which alumina is most preferable. The present catalyst can be used upon coating onto a substrate. Cordierite is a most suitable substrate, but even a metal such as stainless steel can be used as well.
The NOx elimination catalyst can be used in any form, for example, honeycomb form, pellet form, plate form, granule form, powder form, etc., among which the honeycomb form is most preferable.
The present catalyst has an effect of eliminating NOx contained in the exhaust gases emitted from internal combustion engines during the lean-burn operation with high elimination efficiency. The NOx elimination effect of the present catalyst seems due to an action of capturing NOx contained in the lean-burn exhaust gas on the catalyst surface, thereby eliminating NOx from the exhaust gas and an action of reducing the captured NOx, thereby eliminating it. Capturing of NOx seems to be effected by absorption, chemisorption, etc.
The exhaust gas during the lean-burn operation contain oxygen at a higher concentration than the stoichiometric amount necessary for complete combustion of reducing components (HC and CO) in the exhaust gas and thus is in an oxidizing atmosphere. The exhaust gas in an oxidizing atmosphere will be hereinafter referred to as xe2x80x9clean exhaust gasxe2x80x9d or xe2x80x9cair/fuel ratio-lean exhaust gasxe2x80x9d. An exhaust gas emitted from internal combustion engines upon combustion in a theoretical air/fuel ratio will be hereinafter referred to as xe2x80x9cstoichiometric exhaust gasxe2x80x9d or xe2x80x9cair/fuel ratio-stoichiometric exhaust gasxe2x80x9d. An exhaust gas emitted from internal combustion engines operated in fuel excess over the theoretical air/fuel ratio contains oxygen at a lower concentration than the stoichiometric amount necessary for complete combustion of the reducing components contained in the exhaust gas and thus is in a reducing atmosphere. The exhaust gas in a reducing atmosphere will be hereinafter referred to as xe2x80x9crich exhaust gasxe2x80x9d or xe2x80x9cair/fuel ratio-rich exhaust gasxe2x80x9d.
One embodiment of the present invention provides an exhaust gas purification apparatus provided with said catalyst in the exhaust gas passage from internal combustion engines in lean-burn operation.
Another embodiment of the present invention provides an exhaust gas purification process for purifying an exhaust gas emitted by lean-burn operation upon contacting the exhaust gas with said catalyst.
When the present catalyst is kept in continuous contact with an air/fuel ratio-lean exhaust gas, the NOx elimination efficiency will be gradually lowered, because the amount of captured NOx on the catalyst surface gradually increases, thereby weakening the capturing action. When the NOx elimination efficiency is so lowered, it is desirable to temporarily shift the lean-burn operation to a theoretical air/fuel ratio operation or a fuel excess operation, thereby bringing the air/fuel ratio of the exhaust gas into a stoichiometric or fuel-rich exhaust gas, the NOx elimination action can proceed so actively that the NOx captured on the catalyst surface can be rapidly eliminated to regenerate the catalyst. Thus, when the operation is shifted again to the lean-burn operation once again, high NOx elimination efficiency can be obtained. The duration of the theoretical air/fuel ratio or fuel excess operation is only a few seconds to a few minutes.
The present catalyst can be used in combination with a combustion catalyst capable of combusting HC and CO. For the combustion catalyst capable of combusting HC and CO, it is desirable to use a catalyst comprising Pt and Rh supported on an alumina carrier or a catalyst comprising Ag and Mn supported on an alumina carrier. The combustion catalyst can be provided at a position upstream or downstream of the present catalyst or at both upstream and downstream positions.